In lithography, there is an ongoing desire to reduce the size of features in a lithographic pattern in order to increase the density of features on a given substrate area. In photolithography, the push for smaller features has resulted in the development of technologies such as immersion lithography and extreme ultraviolet (EUV) lithography, which are, however, rather costly.
A potentially less costly road to smaller features that has gained increasing interest is so-called imprint lithography, which generally involves the use of a “stamp” (often referred to as an imprint template or an imprint lithography template) to transfer a pattern onto a substrate. An advantage of imprint lithography is that the resolution of the features is not limited by, for example, the emission wavelength of a radiation source or the numerical aperture of a projection system. Instead, the resolution is mainly limited to the pattern density on the imprint lithography template, and the dimensions of pattern features constituting that pattern.
Imprint lithography involves the patterning of an imprintable medium on a surface of a substrate to be patterned. The patterning may involve bringing together a patterned surface of an imprint lithography template and a layer of imprintable medium (e.g., moving the imprint lithography template toward the imprintable medium, or moving the imprintable medium toward the imprint lithography template, or both) such that the imprintable medium flows around and about protrusions on, or recessed in, the patterned surface, to adopt the topography of that patterned surface. The protrusions define pattern features of the patterned surface of the imprint template. Typically, the imprintable medium is flowable when the patterned surface and the imprintable medium are brought together. Following patterning of the imprintable medium, the imprintable medium is suitably brought into a non-flowable or frozen state (i.e. a fixed state), for example by illuminating the imprintable medium with actinic radiation such as UV radiation. The patterned surface of the imprint lithography template and the patterned imprintable medium are then separated. The substrate and patterned imprintable medium are then typically processed further in order to pattern or further pattern the substrate. The imprintable medium may be provided in the form of droplets (e.g. deposited by ink jet printing) on the surface of a substrate to be patterned, but may alternatively be provided using spin coating or the like.
As with any form of lithography, it is desirable to be able to apply patterns to a substrate in an accurate and consistent manner. In some instances, it may be required to align the application of a pattern on top of and relative to a previously applied (and/or processed) pattern. This is known as an overlay requirement. It becomes increasingly difficult to meet these pattern application requirements and/or overlay requirements if there are positional errors of, or in the measurement of, the imprint lithography template with respect to a target portion (which includes an area, region, or the like) of a substrate.